1. Field of the Invention
The present invention relates generally to the field of producing and applying electrical energy to tissues for the purpose of stimulating such tissues, and to therapeutic methods and apparatus. More specifically, the present invention relates to a system for providing and using electrical signals having characteristics that allow them to minimize electrical impedance of tissues, and more specifically yet, to various methods and apparatus for providing such signals for the purpose of providing therapeutic benefit to a living being.
2. Description of Related Art
A number of examples of recent patents may be used to understand the background of the present invention, as well as some of the limitations it overcomes. For example, Boveja, et al. in U.S. Pat. No. 7,076,307 disclose a method for pulsed electrical stimulation of the vagus nerve as a means of providing therapy for a number of neurological disorders. As another example, Whitehurst, et al., disclose in several patents (U.S. Pat. No. 7,013,177; U.S. Pat. No. 6,950,707; U.S. Pat. No. 6,922,590; U.S. Pat. No. 6,901,294; and U.S. Pat. No. 6,871,099) methods for treating a number of disorders involving the application of electrical stimulation to the brain and/or the spinal cord. In these patents, the method of delivery for the electrical stimulation signal involves surgically implanting some device within the tissues. The disclosure of King in U.S. Pat. No. 6,745,079 is yet another example of teachings involving implantation for electrical stimulation. However, King teaches the use of implantable electrodes associated with an external device.
More general examples of patents exist that describe the benefit of electrical stimulation of tissues. These examples include the teachings of Carter (U.S. Pat. No. 6,853,863) and Borkan (U.S. Pat. No. 6,662,053). In U.S. Pat. No. 7,054,686, MacDonald discloses a process for stimulating tissue, such as cardiac tissue, nerve tissue, and brain tissue, by delivering a sequence of individual pulses. George, et al., disclose in U.S. Pat. No. 7,024,239 a method of using electromagnetic energy as a form of tissue stimulation for the purposes of treating chronic wounds. Similar teachings exist for the use of electrical stimulation in speeding the healing process of wounds, and in particular, to the repair process of injured bones (U.S. Pat. No. 6,858,000 by Schukin, et al. and U.S. Pat. No. 6,678,562 by Tepper, et al.).
Among other things, none of these patents discloses any consideration of the electrical nature of the tissues themselves, either at the macro level or at the basic cellular level, or of how tissue stimulation signals might be adapted to take into account that electrical nature. At the macro level, a reduction of the fundamental impedance of tissues will have the effect of providing for increased conductance and hence deeper penetration of an applied electrical signal or field into said tissues. At the cellular level, impedance changes similarly affect conductance, and also in the case of neural cells, probably affect electrical properties such as nerve conduction velocity and neuron firing rates. As with all materials that have the ability to conduct electricity, the impedance of tissues involves components of both resistance and reactance. Generally speaking, tissue is a relatively poor conductor of electricity due to high resistance values. However, tissues also have a capacitive nature that provides for a form of impedance formally known as capacitive reactance.
Capacitive reactance decreases as the frequency of an electrical signal increases. This principle is the basis for the general knowledge that an ideal capacitor will completely block a zero-frequency signal (also known as a “DC” signal) since the capacitor's capacitive reactance will be infinite. Similarly, the same capacitor will pose very little impedance to a signal of very high frequency. Considering the capacitive nature of tissues, higher frequency signals are more readily conducted through them.
However, for the purposes of affecting tissues in a therapeutic way, lower frequency signals are relevant. Thus, a paradox exists in the pursuit of the use of electricity for therapeutic purposes in that, while the low frequency signals are useful for affecting tissues and biochemicals, they are also most severely attenuated by tissue impedance.
The patents discussed above generally attempt to overcome this by using implantable devices that place the source of the electrical stimulating energy in close proximity to the tissues meant to be stimulated, or by providing stimulating electricity at levels that are sufficiently high to allow for attenuation and still accomplish an effect. In the latter case, the comfort of the subject receiving the stimulation electricity is frequently compromised during therapy.